Ba-rich celestine: new data and crystal structure refinement

Ba-rich celestine: new data and crystal structure
refinement
M. F. BRIGATTI,E. GALL1AND L. MEDICI
Dipartimento di Scienze della Terra, Universith di Modena, Largo S. Eufemia 19, 1-41100, Modena, Italy
Abstract
A Ba-rich celestine (Sr0.87Bao.~3SO4) filling cavities of volcanoclastic rocks from Montecchio Maggiore
(Vicenza, Italy) was studied. The role of the Ba content in the mineral was determined using X-ray powder
data, single crystal X-ray refinement, thermal and chemical analyses. The unit cell parameters (obtained by
single crystal diffraction) are a = 8.408, b = 5.372, c = 6.897 A, and the refinement in the space group Pnma
(Z = 4) gives the final R value of 0.039. The average (Sr,Ba)-oxygen bond length is 2.842 A and agrees with
an occupancy of Sr 87% and Ba 13%. Individual bond lengths (Sr,Ba)-O and bond strength calculations
confirm that all twelve interactions are significant and define an irregular array around the cation.
KEYWORDS: celestine, barium, crystal structure refinement, Vicenza, Italy.
Introduction
THE strontium-barium sulphate system (celestinebaryte) has been extensively studied. It is known that
a solid-solution series is formed over the entire
compositional range (Bostrom et al., 1968; Browner,
1973) and that the unit cell edges vary non-linearly
with composition (Bostrom et al. 1968; Burkhard,
1973; Goldish, 1989). Furthermore, a statistical study
on naturally-occurring samples shows that the
composition of the great majority of mixed
celestine-baryte crystals is close to that of the end
members, and ranges between Bal.o-Sro.lBao.9 and
between SrLo--Sro.pBao.1 (Hanor, 1968; Bouhlel,
1985),
The end-member celestine structure type was first
determined by James and Wood (1925) and later
refined by Garske and Peacor (1965), Hawthorne and
Ferguson (1975) and Miyake et al. (1978), but the
role of Ba in naturally occurring samples has not
been thoroughly investigated. Here we present the
mineralogical and structural details of a natural Barich celestine (Ba = 0.13 atoms per formula unit).
Occurrence, properties and composition
The occurrence of celestine in basalts and in
volcanoclastic sedimentary rocks outcropping near
Montecchio Maggiore, Vicenza, north-eastern Italy,
has already been reported (Billows, 1920; Boscardin
and Sovilla, 1988). The mineral fills rock cavities and
occurs as euhedral to subhedral crystals, as crystal
fragments and as granular aggregates. It is white or
light blue in colour and closely associated with
analcite, natrolite, celadonite and apophyllite.
Recently, a number of samples of celestine have
been lound in a slope-cutting carried out in basaltic
lava, near the town. It occurs rarely as single crystals
(Fig. la), almost always as aggregates of subparallel
lamellae from colourless to snow-white, which
decussate in an irregular manner to fill the rock
cavities (Fig. lb).
The crystal density, measured at room temperature
by the Berman method (1939) is 4.046(3) g/cm 3 and
agrees with a composition of Sro.87Bao.13SO4.
Backscattered electron imaging on several crystal
fragments (using a Philips SEM XL-40 equipped
with energy dispersive electron microprobe EDSEDAX 9900) did not reveal Sr or Ba zoning. The
quantitative chemical data gathered by a wavelength
dispersive ARL-SEMQ electron microprobe
(operating voltage of 15 kV, 15 nA sample current,
5 pm electron beam diameter; ~(pZ) correction
procedures; after Pouchou and Pichoir, 1985), on the
same crystal used for the structure refinement, are
reported in Table 1.
Thermogravimetric analysis (SEIKO 5200 apparatus; start temperature: 20~
limit temperature:
1300~ program rate: 10~
Ar gas, flow rate:
30 ml/min) revealed only one weight loss, linked to a
broad derivative maximum at 1200~ which is
produced by SO3 loss.
Mineralogical Magazine, June 1997, Vol. 61, pp. 447~t51
9 Copyright the Mineralogical Society
448
M. F. BRIGATTI ETAL.
TABLE 1. Representative electron microprobe analysis
and chemical formula of Ba-rich celestine from
Montecchio Maggiore (Vicenza, Italy)
Chemical data
(oxide wt.%)
SrO
BaO
SO3
47.41
10.49
42.11
Sum
100.01
Atoms per formula unit
Sr
Ba
S
0.87
0.13
1.00
Collection of Xray single-crystal data and
structure refinement. For intensity data collection a
F~G. 1. Scanning electron micrographs showing: a) Barich celestine crystals and b) the crystal nets. The
photographs were taken using a Philips SEM XL-40
scanning electron microscope.
X-ray crystallography
X-ray powder diffraction. X-ray powder diffraction data sets were collected using an automated
Philips diffractometer, Cu-K~ Ni-filtered radiation
with PbNO3 as an internal standard.
The unit cell parameters based on orthorhombic
indexing (space group Pnma) are: a = 8.414(3), b =
5.369(2), c = 6.909(3) A, and were derived from
d-spacing by a least squares refinement program
(Alberti, 1976). Goldish (1989) emphasized that
synthetic solid solution unit cell parameters show
deviations from Vegard's Law, concerning mostly
the c parameter, and he therefore suggests that the
SrSO4 molar fraction can be derived from the
equation d(2~) = 3.1023 - 0 . 1 3 1 7 •
(where S is
the SrSO4 mole fraction). Our data agree with the
composition Sr0.ssBa0.12SO4, which in turn agrees
very well with the one determined by chemical
analysis.
plate crystal measuring 0.17 [100] • 0.04 [010] •
0.11 [001] mm was mounted on a Siemens P4
rotating anode automated four-circle diffractometer
(Mo-Kct graphite monochromatized radiation operating at 45 kV, 140 mA), equipped with XSCANS
software (Siemens, 1993). Cell dimensions were
refined using the 20 values for 30 centred reflections
with 25 ~< 20 ~< 45. Intensities for reflections
+_h+k+l were collected to a maximum value of 70 ~
using the 03 scan mode, with a scan width of 1.6 ~ and
corrected for background, Lorentz, polarization and
absorption effects. Intensity data of symmetrically
equivalent reflections were averaged. Three standard
reflections measured every 1 h showed less than
1.0% variation in intensity.
Structure refinement was carried out using 596
unique reflections (I ~> 3~(I), Rin t = 0 . 0 2 5 ) with the
SHELX76 and SHELX-S86 programs (Sheldrick,
1976, 1986), starting from the atomic parameters
given by Pasero and Davoli (1987) for isostructural
hashemite in the centrosymmetric space group Pnma.
The final agreement factors were R = 0.039 and wR =
0.037. The structure refinement in the non-centrosymmetric space group Pn21a produced neither an
improvement in the R agreement value nor a
significant departure of the atoms from the position
on the mirror plane. The e.s.d.'s on bond distances
and angles were computed using the program ORFFE
(Busing et al, 1962). The atomic scattering factors for
neutral atoms were taken from the International
Tables for X-ray Crystallography (Ibers and
Hamilton, 1974). Fractional atomic coordinates,
isotropic and anisotropic thermal factors are reported
in Table 2, selected bond distances and angles are
reported in Table 3. The observed and calculated
structure factor list is available on request.
Thermal factors are generally lower than those
previously reported for the two end-members SrSO4
and BaSO4. This feature could be ascribed to
experimental conditions. Furthermore, they confirm
Ba-RICH CELESTINE
449
TABLE 2. Single-crystal data; lattice constants, atom coordinates and displacement factors (~2) of Ba-rich
celestine from Montecchio Maggiore (Vicenza, Italy)
Cell parameters (~,)
a
b
c
8.408(2)
5.372(1)
6.897(2)
Atomic coordinates and isotropic thermal factors.
Atom
M
S
Ol
02
03
x/a
0.1836(1)
0.4377(2)
0.5939(5)
0.3083(5)
0.4220(3)
y/b
0.25
0.75
0.75
0.75
0.9760(5)
Beq
z/c
0.1585(1)
0.1851 (2)
0.0945(8)
0.0437(7)
0.3106(5)
0.66(1)
0.32(3)
1.5(1)
1.1(1)
0.80(9)
Anisotropic thermal factors.
Atom
M
S
O1
02
03
~11
0.39(1)
0.23(4)
0.9(1)
0.8(1)
1.0(1)
~22
0.87(2)
0.24(4)
1.9(2)
1.6(2)
0.43(8)
~33
0.72(1)
0.47(4)
1.6(2)
0.9(1)
0.9(1)
~12
0
0
0
0
0.20)
~13
-0.03(2)
-0.05(3)
0.5(1)
-0.4(1)
-0.16(9)
1323
0
0
0
0
-0.29(9)
The form of the anisotropic thermal parameter is:
exp[- 1/4([31~,h2a*Z+~22kZb*Z+[33312c2+21312hka*b*+21313hla*c*+2[~z3klb*c*)].
the trend previously reported for celestine and baryte:
isotropic thermal factors of oxygen atoms lying on
the mirror plane (O1 and 02) are significantly higher
than that of 03 in general position (Hill, 1977).
The structure of Ba-rich celestine consists of
isolated (SO4) tetrahedra linked by Sr (or Ba)
twelve-coordinated atoms. This implies that every
O atom is fourfold coordinated and linked to three Sr
(or Ba) atoms and to one S atom.
The tetrahedral mean bond length (1.477 A) is
close to the mean bond distances reported both for
celestine (1.473 A, Hawthorne and Ferguson, 1975;
1.474 A, Miyake et al., 1978) and baryte (1.476 A,
Hill, 1977; 1.478 A, Miyake et al., 1978) and to the
mean sulphur-oxygen distance of 1.473 A after Baur
(1970) for sulphate structures. Furthermore, Ba-rich
celestine shows a small increase in tetrahedral
distortions (TAV = 2.58, BLDT = 1.34%) with
respect to those observed both for celestine and for
baryte. According to Hawthorne and Ferguson
(1975), this indicates that tetrahedral distortions are
a function of the geometry of the structure rather than
the chemistry of the twelve-coordinated site.
The metal-oxygen mean bond length decreases
according to the ionic radius of the twelve
coordinated cation M, from baryte o(<M-O> =
2.952 A, Hill, 1977; < M - O > = 2.952 A, Miyake et
al., 1978) to celestine ( < M - O > = 2.827 A,
Hawthorne and Ferguson, 1975; <M--O> = 2.831
A, Miyake et al., 1978). In Ba-rich celestine the
< M - O > bond length agrees with a site occupancy of
Sr 87% and Ba 13% (ionic radii by Shannon, 1976).
The site occupancy was also confirmed by the
evaluation of the ionic radii and cell volume, as
suggested by Hawthorne and Ferguson (1975) (V j/3 =
6.783 A, rM = 1.462 A).
The bond strengths for Ba-rich celestine, calculated using the curves and parameters given by
Brown and Shannon (1973) and Brown and Wu
(1976), compared with those of the end-member
celestine (Hawthorne and Ferguson, 1975), confirm
that all twelve M - O interactions are significant: the
two longest M - O interactions also have to be taken
into account, as indicated by the bond strength sums
around O1 and M (Table 4).
Acknowledgements
The authors are indebted to Mr A. Zordan for making
specimens available for investigation and to Dr P.L.
450
M. F. BRIGATTI E T A L .
TABLE 3. Selected bond-lengths and angles from structure refinement o f Ba-rich celestine from
Montecchio Maggiore (Vicenza, Italy)
SO42- group
S - O(1)
S - 0(2)
S - 0(3)
<S - O>
Metal i o n - o x y g e n polyhedra
M = Sr, Ba
(6)
(A)
( x 2) (,~,)
(A)
M - O(1)
M - O(1)
M - 0(2)
M - 0(2)
M - 0(3)
M - 0(3)
M - 0(3)
<M - O>
1.454(5)
1.461(5)
1.497(3)
1.477
O(1) - 0(2) (A)
O(1) - 0(3) ( x 2) (6)
0(2) - 0(3) ( x 2 ) (A)
O(3)' - 0(3)" (A)
<O - O> (A)
2.427(6)
2.405(5)
2.403(5)
2.428(6)
2.412
(,~)
( x 2)
(A)
( x 2)
( x 2)
( x 2)
( x 2)
(A)
(A)
(6)
(A)
(A)
(A)
2.558(5)
3.269(3)
2.658(5)
2.990(2)
2.655(3)
2.699(3)
2.832(3)
2.842
Selected distorsion parameters.
O(1) - S - 0(2) (~
O(1) - S - 0(3) ( x 2) (~
0(2) - S - 0(3) ( x 2) (~
O(3)' - S - 0(3)" (~
<O - S - O> (~
~" BLD (bond length distorsions)
*t ELD (edge length distorsions)
112.7(3)
109.2(2)
108.7(2)
108.4(3)
109.5
:
=
BLDT ~ (%)
ELDT ~ (%)
TAV t (~
BLDH21* (%)
1.34
0.44
2.58
6.74
l no o vZ,~ ,ni = I I ( x - o(X_O)m
) ~ - ( x - o ) , , l , ~"7o
10ov, n I(o-o)~-(o-o),01
n
Z.~i=l
(O
O)m
(Renner and Lehmann, 1986; Kunz et al., 1991).
TAV (tetrahedral angle variance) = ~'~=l ( 0 i - 109.47)2/5 (Robinson et al., 1971).
Fabbri for obtaining scanning electron microprobe
images. Financial support was provided by the
Ministero dell'Universit5 e della Ricerca Scientifica
(MURST) and Consiglio Nazionale delle Ricerche
(CNR) of Italy. CNR is also acknowledged for the
use o f the A R L - S E M Q electron microprobe at the
Dipartimento di Scienze della Terra of Modena
University.
TABLE 4. Bond strengths (l~ for Ba-rich celestine compared with those reported for celestine by Hawthorne and
Ferguson (1975)
S
This study
M
N*
Hawthorne and Ferguson (1975)
S
M
E*
O1
1.599
0.309
0.056( x 2)
2.020 (1.908)*
1.512
0.325
0.054( x 2)
1.945 (1.837)*
02
1.566
0.236
0.104( x 2)
2.010
1.589
0.244
0.098( x 2)
2.029
03
1.411( x 2)
0.238( x 2)
0.212( x 2)
0.152( x 2)
2.013
1.469( x 2)
0.206( x 2)
0.226( x 2)
0.148( x 2)
2.049
Z*
5.987
2.069 (1.957)*
6.039
2.033 (1.925)*
(i) Calculated from the curves and parameters of Brown and Shannon (1973) and Brown and Wu (1976), considering
the linear relation between m e t a l ~ x y g e n bond lengths and SrSO 4 molar fraction.
* Values in parenthcses are for a M coordination of [10].
Ba-RICH CELESTINE
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[Manuscript received 5 March 1996:
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